A standard electro-plating process involves applying a current density of about 3×102 Amps meter−2 in an electroplating bath containing an electrolyte, typically resulting in the deposition on a cathode of a thickness of copper of about 6×10−7 meters minute−1.
Various attempts have been made to improve the deposition process, for example by the use of a rotating disc electrode. At best, such attempts have resulted in increases of up to three times in the deposition thickness by allowing an equivalent increase in current density. A major problem associated with electroplating, especially when high deposition rates are attempted, is the irregularity of deposition. Another major problem is the need for all areas that are to be plated to be electrically connected.
To obtain a uniform plating deposit using existing methods, the required situation is that given by two parallel, co-axial and equi-potential conducting planes separated by a medium of homogenous resistance. If a potential difference exists between the two planes, then the current will flow between and normal to the two planes with uniform density (see FIG. 8). If the medium separating the two planes is an electrolyte of suitable composition containing adequate and suitable ions of the material to be deposited, then a uniform deposition of the material will be made on the plane which is at the more negative potential. The amount of the deposit is dependent upon the material type and the total electrical charge.
In practice, the situation described above does not occur, due to surface roughness of the two planes and the lack of homogeneity of the electrolyte. Also, practical difficulties, associated with achieving true parallelism of the planes and the possible irregular pattern of the conductive surface of the negative (target) plane and the restrictions of the electrolyte flow, to some or all of the target plane surface, add to the lack of uniformity of the current density within the electrolyte. This results in irregular deposits of material on the target surface.
FIG. 9 shows the distortion of the current stream, and therefore current density distribution, due to the irregularity of the target (negative) surface. Further distortions due to the irregularities in the positive surface and variations in the electrolyte resistance are not shown. FIG. 10 shows the accentuation of the irregularities in the target surface due to the unequal current density distribution. The interaction of unequal current density and surface irregularity can be seen to be mutually progressive.
Several techniques have been employed to offset these effects including the use of current diversions (robber bars) at the target surface. Such techniques are only partially successful and are inherently inefficient. There are few, if any, practical techniques for dealing with situations in which the target surface has areas which are to be plated but which are not electrically connected.